The invention herein discloses algorithms for applying Lithographic Proximity Corrections (LPC) to very large scale integrated (VLSI) circuit design databases to compensate for 2-dimensional (2-D) lithographic errors.
LPC attempts to overcome a shortcoming in lithography that prevents the accurate printing of shapes in a design database when the minimum dimensions of the shapes are approximately the same as or below the wavelength of exposure. 2-D lithographic proximity effects occur in mask manufacture, because of electromagnetic diffraction from orthogonal edges of a mask feature, and 2-D chemical effects in the photoresist. The results are generally undesirable. For example, corners become rounded and narrow line ends are shortened. The effects can be overcome by modifying the shapes from their original design in ways that correct the undesirable lithographic transformations that will take place during printing. Thus, even though a corrected mask doesn't resemble an intended design, when the corrected mask is finally printed on a wafer, the wafer pattern better matches the intended design.
FIG. 1 is photolithographic simulation of how a rectangle 82 prints using a deep ultra-violet (UV) stepper, when the rectangle feature size is similar to the UV wavelength used. Such a rectangle 82 prints as an oval 80. This illustrates the line end shortening discussed above.
FIG. 2 is a photolithographic simulation of the same rectangle 82 that has square serifs 90, 92, 94, 96 placed on the four corners of the rectangle 82. The result of adding these square serifs 90, 92, 94, 96 to an initial rectangular shape 82 is that the printed shape 98 more closely matches the shape of the original rectangle 82.